Frequently used for fill level- or limit level measurement in liquid media are measuring devices based on the capacitive measuring principle. Besides fill level, respectively limit-level, such measuring devices can also be used for determining and/or monitoring electrical conductivity and/or permittivity of a medium. For this, the fill level must remain unchanged. Furthermore, capacitive measuring devices can detect accretion formation on measuring apparatus components in contact with the medium.
Capacitive measuring devices comprise a measuring probe having, as a rule, a rod-shaped sensor electrode and, in given cases, a guard electrode for improving the accuracy of measurement in the case of accretion formation on the probe. The fill level of the medium is ascertained from the capacitance of the capacitor formed by the probe electrode and the container wall or a second electrode, wherein an alternating voltage is placed on the probe electrode. Depending on conductivity of the medium, the medium and/or the probe insulation forms the dielectric of the capacitor. The guard electrode lies at the same potential as the probe electrode and surrounds the probe electrode at least sectionally coaxially. A probe with guard is described, for example, in German patent, DE 3212434 C2, while a probe without guard is described in published international application, WO 2006/034959 A2. Capacitive probes for continuous fill level- or limit level determination are produced and sold by the applicant in different embodiments and with different probe lengths, for example, under the mark, LIQUICAP.
In the case of capacitive probes, the following dilemma presents itself: In order to prevent resonance effects, which disturb the measuring, it is advantageous to select the signal frequency of the alternating voltage placed on the measuring probe smaller, the longer the probe is. Higher signal frequencies have, however, the advantage that their sensitivity to accretion formation is less than at lower signal frequencies.
In order to provide an electronics unit compatible for probes of any length, consequently, in the case of known capacitive measuring devices, a signal frequency is used, often also referred to as the measuring frequency, which seems suitable for all probe lengths. Therefore, especially the signal frequency in the case of shorter probes lies below the signal frequency optimal for these shorter probes.
A further problem arises, when the medium has a conductivity, which lies in a transitional region dependent on the permittivity (dielectric conductivity) or independent of the permittivity. The fill level, respectively limit-level, is in this transitional region not reliably determinable by means of a capacitive measuring device. As a result, capacitive fill level measurement is not applicable in the case of such a medium.
Known from published international application, WO 2012/100 873 A1 is an apparatus for capacitively determining and/or monitoring the fill level of a medium in a container. This apparatus provides a solution for the above related problems. The known capacitive measuring device includes a probe unit having at least one probe electrode and an electronics unit. The electronics unit supplies at least the probe electrode with an electrical, transmission signal. Then, the electronics unit receives the electrical response signal from the probe unit and evaluates it. Especially, the probe electrode is successively supplied with measurement signals, which comprise sequential, discrete signal frequencies of a defined frequency range. Based on a frequency sweep, the electronics unit ascertains—as a function of the current application parameters in the process—the optimal signal frequency, respectively measuring frequency. The application parameters include, for example, probe length, possible accretion formation on the probe, character of the medium, etc. Then, the electronics unit determines from the response signal to the measurement signal with the optimal signal frequency the fill level, the limit-level or other physical parameter of the medium.
Commercially obtainable capacitive measuring devices have never had a frequency sweep capability. The known “static” measuring devices use for producing the basic signal a rectangular signal produced by a timer of the microcontroller. Using a higher order, lowpass filter, the harmonic waves of the rectangular signal are so strongly attenuated that there appears on the output of the low pass filter a sine signal, which is sent to the measuring probe via a driver stage. Measured are the amplitude and the phase of the response signal, thus of the alternating current, which flows through the measuring probe. This known variant is usually operated with a signal frequency between 30 kHz and 5 MHz.
Furthermore, Endress+Hauser sells capacitive measuring devices, which use a quartz oscillator as sine generator. In such case, the oscillation frequency of an oscillatory circuit is determined and held stable by the eigenfrequency of the quartz, respectively a multiple thereof. In the per thousand range, this frequency can be changed via a tuning capacitor; it is, however, not possible to change the oscillation frequency over an extended frequency range.
Another variant, which is applied in current measuring devices of Endress+Hauser, is based on an ASIC. For evaluating the sensor, only a few different frequencies in a limited frequency range can be selected, which, however, are not changeable during runtime.
Known from published international application, WO 29010/0139 508 A1 is a method for determining or monitoring with an oscillatable unit a predetermined fill level, a phase boundary or the density of a medium in a container. By means of a frequency sweep, the oscillatable unit is excited to execute oscillations within a predeterminable frequency range in the working range of the oscillatable unit successively with discrete exciter frequencies following one after the other, wherein the corresponding oscillations of the oscillatable unit are received in the form of received signals. Via the frequency sweep, that exciter frequency is ascertained, at which the oscillatable unit oscillates with an oscillation frequency, which has a predetermined phase shift between the transmission signal and the received signal, and wherein the transmitting/receiving unit (S/E) excites the oscillatable unit with the ascertained signal-, respectively oscillation frequency, to execute oscillations or wherein the next frequency sweep is started. Vibronic sensors, which can be operated with the above described method, are manufactured and sold by Endress+Hauser under the marks, LIQUIPHANT and SOLIPHANT. The signal generator of the invention can likewise be used in vibronic sensors.
For implementing dynamic signal generators, switched capacitor filters (also known as SC filters) can be used. In such case, harmonic waves are filtered from a rectangular signal to produce a sine signal, whose signal frequency is set via the clock source: If the clocking frequency changes, the signal frequency and the transfer function of the filter change. In order to suppress the aliasing effect at different signal frequencies, the limit frequency of the analog lowpass filter at the output of the signal generator must be changed further. Preferably, this occurs using a digital potentiometer. The advantage of this known signal generator is that with few components a sine signal with little harmonic distortion and variable frequency can be provided. The disadvantages of the known solution include that a variable clock signal producer is required. Necessary for operating the SC filter are clocking frequencies, which are much higher than the signal frequency. Corresponding solutions require relatively much energy. The factor lies in a range from 50 to 100 (compare e.g. Sandler, H. M., Sedra, A. S.: Sine wave generation using a high-order lowpass switched-capacitor filter. IEEE. April 1986).